File Transfer Protocol FTP File Transfer Protocol

FTP FTP FTP connections Data connection Modes: File Transfer Protocol Used to transfer data from one computer to another over the internet. Client-Server Architecture. FTP connections Control connection Created when an FTP session is established Only for passing control information Data connection Each time that data is sent, a distinct TCP data connect is established Data connection Modes: Active Mode Passive Mode

FTP FTP RFCs: RFC 959 – File Transfer Protocol RFC 2228 – FTP Security Extensions RFC 2428 – FTP Extensions for IPv6 and NATs RFC 2640 – UTF-8 support for file name

FTP – Flow (1) Client Server Binding on port 21 Connect to server port 21 from port A. USER #### PASS ******** EPRT |1|ip|portnum| Send some requests get return data from portnum Quit Server Binding on port 21 Accepts connection from client, output welcome messages. 331 User name okay, need password. 230 User logged in, proceed. 200 PORT Command successful. Binding source port 20, connect to client port portnum, send data. …

FTP – Flow (2) Example Control Connection % telnet freebsd.cs.nctu.edu.tw 21 Trying 140.113.17.209... Connected to freebsd.cs.nctu.edu.tw. Escape character is '^]'. 220---------- Welcome to Pure-FTPd [privsep] ---------- 220-You are user number 7 of 1000 allowed. 220-Local time is now 16:25. Server port: 21. 220-Only anonymous FTP is allowed here 220-IPv6 connections are also welcome on this server. 220 You will be disconnected after 15 minutes of inactivity. USER ftp 331 Any password will work PASS ftp 230 Any password will work EPRT |1|140.113.235.135|65000| 200 PORT command successful list 150 Connecting to port 65000 226-Options: -l 226 2 matches total quit 221-Goodbye. You uploaded 0 and downloaded 0 kbytes. 221 Logout. Connection closed by foreign host. Example Control Connection

FTP – Flow (3) Example (contd.) Retrieving Data Client must bind the random port % nc -l 65000 drwxr-xr-x 852 888 2010 80328 Mar 28 11:39 distfiles drwxr-xr-x 16 888 2010 34 May 11 2008 pub

FTP – commands, responses USER username PASS password LIST Return list of file in current dir. CWD dirname Change working directory RETR filename Retrieves (gets) file. STOR filename Stores (puts) file onto server. EPRT |1|ip|port| Set to active mode PASV(EPSV) Set to passive mode DELE Remove file on the server. QUIT Return Codes First code 1: Positive Preliminary reply 2: Positive Completion reply 3: Positive Intermediate reply 4: Transient Negative Completion reply 5: Permanent Negative Completion reply Second code 0: The failure was due to a syntax error 1: A reply to a request for information. 2: A reply relating to connection information 3: A reply relating to accounting and authorization. 5: The status of the Server file system

FTP – Active Mode vs. Passive Mode (1) FTP client bind a random port (>1023) and sends the random port to FTP server using “EPRT” command. When the FTP server initiates the data connection to the FTP client, it binds the source port 20 and connect to the FTP client the random port sent by client. EPRT |1|ip|port| EPRT |2|ipv6|port| Passive Mode FTP client sends “EPSV/PASV” command to the server, make the server bind a random port (>1023) and reply the random port back. When initializing the data connection, the FTP client connect to the FTP Server the random port, get data from that port. EPSV  Server reply: 229 Entering Extended Passive Mode (|||1868|) PASV  Server reply: 227 Entering Passive Mode (h1,h2,h3,h4,p1,p2) ※ IP:port (6bytes)  h1,h2,h3,h4,p1,p2 Ex. 140.113.17.215:45678  140,113,17,215,178,110

FTP – Active Mode vs. Passive Mode (2)

FTP – When FTP meets NAT/Firewall (1) Firewall behavior Generally, the NAT/Firewall permits all outgoing connection from internal network, and denies all incoming connection from external network. Problem when FTP meets NAT/Firewall Due to the separated command/data connection, the data connections are easily blocked by the NAT/Firewall. Problem Cases: Active mode, NAT/Firewall on client side. Passive mode can solve this problem. Passive mode, NAT/Firewall on server side. Active mode can solve this problem. Both client side and server side have NAT/Firewall The real problem.

FTP – When FTP meets NAT/Firewall (2) Active mode, NAT/Firewall on client side. Passive mode can solve this problem. Client Server NAT/Firewall PORT IP, port Y Connect to port Y BLOCKED Active Mode Client Server NAT/Firewall PASV reply IP, port Z Connect to port Z PASS Passive Mode

FTP – When FTP meets NAT/Firewall (3) Passive mode, NAT/Firewall on Server side. Active mode can solve this problem. Client Server NAT/Firewall PASV reply IP, port Z Connect to port Z BLOCKED Passive Mode Client Server NAT/Firewall PORT IP, port Y Connect to port Y PASS Active Mode

FTP – When FTP meets NAT/Firewall (4) Real Problem: Firewall on both sides. Solution: ftp-proxy running on NAT/Firewall Client Server NAT/Firewall PORT IP, port Y Connect to port Y BLOCKED Active Mode Client Server NAT/Firewall PASV reply IP, port Z Connect to port Z BLOCKED Passive Mode

FTP – Security Security concern Solutions As we seen, FTP connections (both command and data) are transmitted in clear text. What if somebody sniffing the network? We need encryption. Solutions FTP over SSH So called secure-FTP(sftp). Both commands and data are encrypted while transmitting. One connection, but poor performance. FTP over TLS Only commands are encrypted while transmitting. Better performance.

FXP FXP File eXchange Protocol/Proxy FTP A user on one host performs a file transfer from one server to another Two control connections One each from User-PI to the two Server-PI One data connections Server-DTPs are invoked on each server to send data

FTP – Pure-FTPd (1) Introduction A small, easy to set up, fast and secure FTP server Support chroot Restrictions on clients, and system-wide. Verbose logging with syslog Anonymous FTP with more restrictions Virtual Users, and Unix authentication FXP (File eXchange Protocol) FTP over TLS UTF-8 support for filenames

FTP – Pure-FTPd (2) Installation Ports: /usr/ports/ftp/pure-ftpd Options

FTP – Pure-FTPd (3) Startup: Other options WITH_CERTFILE for TLS Default: /etc/ssl/private/pure-ftpd.pem WITH_LANG Change the language of output messages Startup: Add pureftpd_enable=“YES” in /etc/rc.conf

FTP – Pure-FTPd Configurations(1) File: /usr/local/etc/pure-ftpd.conf Documents Configuration sample: /usr/local/etc/pure-ftpd.conf.sample All options are explained clearly in this file. Other documents See /usr/local/share/doc/pure-ftpd/* Randy [/usr/local/share/doc/pure-ftpd] W7 -randy- ls AUTHORS README README.MySQL pure-ftpd.png CONTACT README.Authentication-Modules README.PGSQL pureftpd.schema COPYING README.Configuration-File README.TLS HISTORY README.Contrib README.Virtual-Users NEWS README.LDAP THANKS

FTP – Pure-FTPd Configurations(2) # Cage in every user in his home directory ChrootEveryone yes # If the previous option is set to "no", members of the following group # won't be caged. Others will be. If you don't want chroot()ing anyone, # just comment out ChrootEveryone and TrustedGID. TrustedGID 0 # PureDB user database (see README.Virtual-Users) PureDB /usr/local/etc/pureftpd.pdb # If you want simple Unix (/etc/passwd) authentication, uncomment this UnixAuthentication yes # Port range for passive connections replies. - for firewalling. PassivePortRange 30000 50000 # This option can accept three values : # 0 : disable SSL/TLS encryption layer (default). # 1 : accept both traditional and encrypted sessions. # 2 : refuse connections that don't use SSL/TLS security mechanisms, # including anonymous sessions. # Do _not_ uncomment this blindly. Be sure that : # 1) Your server has been compiled with SSL/TLS support (--with-tls), # 2) A valid certificate is in place, # 3) Only compatible clients will log in. TLS 2 # UTF-8 support for file names (RFC 2640) # Define charset of the server filesystem and optionnally the default charset # for remote clients if they don't use UTF-8. # Works only if pure-ftpd has been compiled with --with-rfc2640 FileSystemCharset big5 # ClientCharset big5

FTP – Pure-FTPd Problem Shooting Logs Location In default, syslogd keeps ftp logs in /var/log/xferlog Most frequent problem pure-ftpd: (?@?) [ERROR] Unable to find the 'ftp' account It’s ok, but you may need it for Virtual FTP Account. pure-ftpd: (?@?) [ERROR] Sorry, but that file doesn't exist: [/etc/ssl/private/pure-ftpd.pem] If you set TLS = 2, then this file is needed. How to generate a pure-ftpd.pem? See README.TLS

FTP – Pure-FTPd Tools pure-* pure-ftpwho pure-pw List information of users who use the FTP server now. pure-pw To create Virtual Users using PureDB pure-pw(8) See README.Virtual-Users

FTP – More Tools ftp/pureadmin ftp/lftp ftp/wget ftp/mget FileZilla Management utility for the PureFTPd ftp/lftp A powerful functional client Support TLS ftp/wget Retrieve files from the Net via HTTP(S) and FTP ftp/mget Multithreaded commandline web-download manager FileZilla An FTP Client for Windows

FTP – PF: Issues with FTP (1) Reference: http://www.openbsd.org/faq/pf/ftp.html FTP Client Behind the Firewall Problem Clients cannot use active mode Use ftp-proxy(8) ftpproxy_enable=“YES” In pf.conf nat-anchor “ftp-proxy/*” rdr-anchor “ftp-proxy/*” rdr on $int_if proto tcp from any to any port 21 -> 127.0.0.1 port 8021 anchor “ftp-proxy/*”

FTP – PF: Issues with FTP (2) PF “Self-Protecting” an FTP Server Problem Clients cannot use passive mode Open holes so that clients can connect into the data channel In pf.conf pass in on $ext_if proto tcp from any to any port 21 keep state pass in on $ext_if proto tcp from any to any port > 49151 keep state

FTP – PF: Issues with FTP (3) FTP Server Protected by an External PF Firewall Running NAT Problem Clients cannot use passive mode Use ftp-proxy(8) Need some flags of ftp-proxy ftpproxy_flags="-R 10.10.10.1 -p 21 -b 192.168.0.1" In pf.conf nat-anchor “ftp-proxy/*” nat on $ext_if inet from $int_if -> ($ext_if) rdr-anchor “ftp-proxy/*” pass in on $ext_if inet proto tcp to $ext_ip port 21 flags S/SA keep state pass out on $int_if inet proto tcp to $ftp_ip port 21 user proxy flags S/SA keep state anchor “ftp-proxy/*”